Electrostatic precipitators fill an invaluable role in reducing air pollution. Primary sources of industrial air pollution include particulate matter from exhaust gases, the combustion of fossil fuels, and chemical processes. An electrostatic precipitator generates a strong electrical field that charges the particulate matter leaving an exhaust stack. These charged particles are then collected prior to leaving the exhaust stack to prevent these particles from polluting the atmosphere.
A conventional single-phase power supply for an electrostatic precipitator characteristically includes an alternating current voltage source of 380 to 600 volts with a frequency of either 50 or 60 Hertz. Typically, silicon-controlled rectifiers, which are controlled by an automatic voltage control device, are used to manage the power and modulate the time the alternating current line input flows to a transformer and a full wave, bridge rectifier. The full wave, bridge rectifier converts the alternating current voltage to a pulsating direct current voltage and doubles the output frequency to either 100 or 120 Hertz, respectively. This high voltage, direct current output is electrically connected to the electrostatic precipitator. There is a low pass filter in the form of a current limiting reactor electrically connected in series between the silicon-controlled rectifiers and the input to the transformer for limiting the high frequency energy and shaping the voltage waveform.
The electrostatic precipitator essentially operates as a capacitor with two conductors separated by an insulator. The conductors are the discharge electrodes and the collecting plates and the insulator is the exhaust gas that is being treated. The electrostatic precipitator essentially performs two functions. The first function is to operate as a load on the power supply so that corona current can be used to collect particles and the second function is to operate as a low pass filter. Since the capacitance of this low pass filter is of a relatively low value, the voltage waveform for the electrostatic precipitator has a significant amount of ripple voltage.
Sparking is a phenomenon that limits the electrical energization of the electrostatic precipitator. This is when the gas that is being treated in the exhaust stack has a localized breakdown so that there is a rapid rise in electrical current with an associated decrease in voltage. Therefore, instead of having the corona current distributed evenly across the entire field for the electrostatic precipitator, there is a high amplitude spark that funnels all of the available current in one path rather than an innumerable number of paths. This can cause damage to the internal components of the electrostatic precipitator as well as disrupt the entire operation of the electrostatic precipitator. The automatic voltage control device operates to interrupt power once a spark is sensed. The current limiting reactor then acts as a low pass filter to choke off the delivery of high frequency energy to the transformer. During this brief quench period, the current dissipates through this localized path of electrical conduction until the spark is extinguished and then the voltage is reapplied.
To improve particle collection efficiency for an electrostatic precipitator, the ripple voltage for the electrostatic precipitator is reduced. This is important since the presence of ripple voltage results in a peak value of the voltage waveform for the electrostatic precipitator that is greater than the average value of the voltage waveform for the electrostatic precipitator. Therefore, since the peak value of the voltage waveform for the electrostatic precipitator must not exceed the breakdown or sparking voltage level due to the problems described above, the average voltage that operates the electrostatic precipitator will be at a lower level. This lower level of average voltage will negatively affect the particle collection efficiency for the electrostatic precipitator.
One method of accomplishing this reduction in ripple voltage involves the complete replacement of the conventional, single-phase power supply with a three-phase, high frequency power supply. This three-phase, high frequency power supply is complex and very expensive, and there is a significant amount of down time when making this substitution. In addition, the three-phase, high frequency power supply typically emits an objectionable squeal from the magnetic components if this power supply is not operated at frequencies above that for human hearing. Operating at these high frequencies generates substantial amounts of heat so that oil pumps, heat exchangers and fans are typically required. Therefore, a failure of the cooling system results in a failure of the three-phase, high frequency power supply. The replacement of a single-phase power supply with a three-phase power supply is very cumbersome and costly since the entire system must be replaced including power cables and switch gear. Although this three-phase, high frequency power supply will provide virtually no ripple voltage and the minimum value, average value, and peak value of the precipitator voltage waveform will remain substantially the same, it is apparent that these previously described disadvantages are significant.
There are other methods of reducing ripple voltage applied to an electrostatic precipitator, such as having variable inductance for the current limiting reactor. However, this modification does not reduce the ripple voltage nearly enough the achieve the desired particulate collection efficiency since in addition to the significant presence of ripple voltage there is also variance between the minimum, average and peak values for the voltage waveform for the electrostatic precipitator.
Another method of reducing the ripple voltage applied to an electrostatic precipitator is to attach a T-type filter or π-type filter between the output of the full wave, bridge rectifier and the electrostatic precipitator field. A significant disadvantage to this system is all power supplied to the electrostatic precipitator must first flow through the T-type or π-type filter. This requires the discharge time constant for the filter to be long compared to the charge time constant for the electrostatic precipitator so that capacitance of the filter must be much larger than the capacitance of the electrostatic precipitator field. There is a well-known negative relationship with capacitance placed in parallel with the electrostatic precipitator. This relationship leads to powerful sparking that, as described previously, can paralyze the operation of the electrostatic precipitator as well as damage the electrostatic precipitator. When the capacitance of the filter is decreased to avoid this problem, then the collection efficiency of the electrostatic precipitator is considerably reduced.
The present invention is directed to overcoming one or more of the problems set forth above.